Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 241: Magnitude of action potential is mainly affected by:

A. Calcium ion
B. Hydrogen ion
C. Sodium ion (Correct Answer)
D. Potassium ion

Explanation: ### Explanation **1. Why Sodium Ion is Correct:** The **magnitude (amplitude)** of an action potential is primarily determined by the concentration gradient of sodium ions ($Na^+$) across the cell membrane. According to the **Hodgkin-Huxley model**, the peak of the action potential approaches the **Equilibrium Potential of Sodium ($E_{Na}$)**, which is approximately $+60\ mV$. * During depolarization, voltage-gated $Na^+$ channels open, allowing a massive influx of $Na^+$. * If the extracellular $Na^+$ concentration decreases (hyponatremia), the concentration gradient weakens, leading to a lower peak and reduced magnitude of the action potential. **2. Why Other Options are Incorrect:** * **Potassium Ion ($K^+$):** $K^+$ primarily determines the **Resting Membrane Potential (RMP)** and the **repolarization** phase. Changes in $K^+$ affect the excitability of the cell (threshold) rather than the peak magnitude of the spike. * **Calcium Ion ($Ca^{2+}$):** Extracellular $Ca^{2+}$ acts as a "gatekeeper" for $Na^+$ channels. Low $Ca^{2+}$ (hypocalcemia) lowers the threshold for firing (causing tetany), but it does not significantly dictate the peak magnitude of the action potential in nerve/skeletal muscle. * **Hydrogen Ion ($H^+$):** pH changes affect overall protein function and excitability (acidosis depresses excitability; alkalosis increases it), but $H^+$ is not a primary charge carrier for the action potential spike. **3. Clinical Pearls for NEET-PG:** * **RMP** is mainly dependent on **Potassium** (due to high resting permeability). * **Action Potential Magnitude** is mainly dependent on **Sodium**. * **Hypocalcemia** increases neuronal excitability (Tetany) by lowering the threshold potential. * **Hyperkalemia** initially increases excitability (brings RMP closer to threshold) but eventually causes inactivation of $Na^+$ channels, leading to paralysis or cardiac arrest.

Question 242: Fast axonal transport is a component of orthograde axonal transport. What is the rate of fast axonal transport?

A. 100 mm/d
B. 200 mm/d
C. 400 mm/d (Correct Answer)
D. 600 mm/d

Explanation: **Explanation:** Axonal transport is the process by which organelles, proteins, and vesicles are moved along the axon. It is categorized based on direction (Orthograde/Anterograde vs. Retrograde) and speed (Fast vs. Slow). **1. Why Option C is Correct:** **Fast Axonal Transport** (specifically the orthograde component) occurs at a rate of approximately **400 mm/day**. This process is mediated by the motor protein **Kinesin**, which "walks" along microtubules using ATP. It is responsible for transporting membrane-bound organelles, mitochondria, and neurotransmitter vesicles from the cell body to the axon terminals. **2. Analysis of Incorrect Options:** * **Options A and B (100–200 mm/d):** These rates are too slow for fast transport. While some sources cite a range (200–400 mm/d), **400 mm/day** is the standard value cited in major physiology textbooks (like Guyton and Ganong) and is the high-yield figure for NEET-PG. * **Option D (600 mm/d):** This exceeds the physiological rate of standard fast axonal transport in human neurons. **3. Clinical Pearls & High-Yield Facts:** * **Slow Axonal Transport:** Occurs at a rate of **0.5 to 10 mm/day**. It transports structural elements like the axoplasmic matrix and cytoskeletal proteins (neurofilaments). * **Retrograde Transport:** Moves from the periphery to the cell body at a rate of **~200 mm/day** via the motor protein **Dynein**. * **Clinical Relevance:** Certain neurotropic viruses (e.g., **Rabies, Herpes Simplex**) and toxins (e.g., **Tetanus toxin**) exploit **Retrograde transport** to reach the Central Nervous System. * **Mnemonic:** **K**inesin moves to the **K**ick-off (Anterograde/Forward); **D**ynein moves to the **D**en (Retrograde/Backward).

Question 243: Among the following nerve fibers, which one has the lowest conduction velocity?

A. A alpha
B. A beta
C. A gamma
D. C (Correct Answer)

Explanation: **Explanation:** The conduction velocity of a nerve fiber is primarily determined by two factors: **myelination** and **fiber diameter**. According to the Erlanger-Gasser classification, nerve fibers are categorized into Types A, B, and C based on these characteristics. **Why Option D is Correct:** **Type C fibers** are the only **unmyelinated** fibers in the human body. They have the smallest diameter (0.4–1.2 μm) and the slowest conduction velocity (0.5–2.0 m/s). Because they lack the insulating myelin sheath, they cannot perform saltatory conduction and instead rely on slow, continuous propagation of action potentials. They primarily transmit "slow" pain, temperature, and post-ganglionic autonomic signals. **Why Other Options are Incorrect:** * **A alpha (Aα):** These are the thickest, most heavily myelinated fibers (12–20 μm) with the fastest conduction velocity (70–120 m/s). They handle proprioception and somatic motor functions. * **A beta (Aβ):** These are large, myelinated fibers involved in touch and pressure. Their velocity (30–70 m/s) is significantly higher than Type C. * **A gamma (Aγ):** These myelinated fibers supply muscle spindles (intrafusal fibers). While slower than Aα and Aβ, their velocity (15–30 m/s) is still much faster than the unmyelinated C fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Local Anesthetics:** Block **Type C** fibers first (smallest diameter), which is why pain is the first sensation lost. * **Hypoxia:** Affects **Type A** fibers first (most metabolically active). * **Pressure:** Affects **Type A** fibers first (e.g., "limb falling asleep"). * **Conduction Velocity Formula:** For myelinated fibers (Type A), Velocity (m/s) ≈ 6 × Diameter (μm). * **Pain Transmission:** **Aδ fibers** carry "fast/sharp" pain, while **C fibers** carry "slow/dull/aching" pain.

Question 244: At the neuromuscular junction:

A. The muscle membrane possesses muscarinic receptors.
B. The motor nerve endings secrete norepinephrine.
C. Curare leads to prolongation of neuromuscular transmission.
D. The motor nerve endings secrete acetylcholine. (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. The motor nerve endings secrete acetylcholine.** At the Neuromuscular Junction (NMJ), the transmission of an impulse from a motor neuron to a skeletal muscle fiber is chemical. When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open, leading to the exocytosis of synaptic vesicles. These vesicles release **Acetylcholine (ACh)**, the primary neurotransmitter at the NMJ, into the synaptic cleft. #### Analysis of Incorrect Options: * **Option A:** The muscle membrane (sarcolemma) at the motor endplate contains **Nicotinic receptors (N$_m$ subtype)**, which are ligand-gated ion channels. Muscarinic receptors are typically found in the autonomic nervous system (parasympathetic effectors). * **Option B:** Motor nerve endings are **cholinergic**, meaning they secrete acetylcholine. Norepinephrine is the primary neurotransmitter for most sympathetic postganglionic neurons, not somatic motor neurons. * **Option C:** Curare is a competitive antagonist that binds to nicotinic ACh receptors. By blocking these receptors, it prevents depolarization, leading to **paralysis** rather than prolongation of transmission. (Note: Acetylcholinesterase inhibitors like Neostigmine are what typically prolong the presence of ACh in the cleft). #### High-Yield Clinical Pearls for NEET-PG: * **Myasthenia Gravis:** Characterized by autoantibodies against the **post-synaptic** nicotinic ACh receptors, leading to muscle weakness that worsens with activity. * **Lambert-Eaton Syndrome:** Caused by antibodies against **pre-synaptic** voltage-gated calcium channels; weakness typically improves with repeated contraction. * **Botulinum Toxin:** Prevents the release of ACh from the presynaptic terminal by degrading SNARE proteins, causing flaccid paralysis. * **Safety Factor:** The NMJ normally releases more ACh than is required to trigger an action potential, ensuring every nerve impulse results in a muscle contraction.

Question 245: What does the 'all or none' phenomenon mean in physiology?

A. The magnitude of a response is related to the strength of the stimulus.
B. An action potential has a fixed, all-or-nothing value. (Correct Answer)
C. Action potentials can be developed by subthreshold stimuli.
D. The magnitude of a response is related to the duration of the stimulus.

Explanation: ### Explanation The **All-or-None Law** is a fundamental principle of neurophysiology. It states that if a stimulus is strong enough to reach the **threshold potential** (typically -55mV), an action potential will be generated at a constant, maximum amplitude. If the stimulus is subthreshold, no action potential occurs at all. **Why Option B is Correct:** Once the threshold is reached, voltage-gated sodium channels open in a regenerative cycle. The resulting action potential is independent of the stimulus strength; its amplitude and shape remain **fixed and constant** for that specific nerve or muscle fiber. Increasing the intensity of the stimulus will not increase the size of the action potential. **Analysis of Incorrect Options:** * **Option A & D:** These describe **graded potentials** (like receptor potentials or EPSPs), where the magnitude varies with the strength or duration of the stimulus. In contrast, action potentials are non-graded. * **Option C:** Subthreshold stimuli only cause local electronic changes (local response) but fail to trigger the rapid depolarization required for an action potential. **High-Yield NEET-PG Pearls:** * **Frequency Coding:** Since the amplitude of an action potential is fixed, the intensity of a stimulus is communicated to the CNS by the **frequency** of action potentials, not their size. * **Applicability:** The All-or-None law applies to **individual** nerve fibers and motor units. It does **not** apply to a whole nerve trunk (which contains many fibers with different thresholds) or a whole skeletal muscle, which show graded responses via recruitment. * **Refractory Period:** This ensures that action potentials remain discrete events, preventing them from merging or summing together.

Question 246: Rheobase indicates:

A. Magnitude of current (Correct Answer)
B. Rate of discharge
C. Velocity of nerve conduction
D. Specificity of impulse transmission

Explanation: ### Explanation **Rheobase** is defined as the **minimum intensity (magnitude) of a constant electrical current** that, when applied for an indefinite period, is required to produce an action potential in an excitable tissue (nerve or muscle). #### Why Option A is Correct: The Strength-Duration curve illustrates the relationship between the intensity of a stimulus and the time required to excite a tissue. Rheobase represents the **threshold intensity** on the Y-axis. If the current magnitude is below the rheobase, the tissue will not fire, regardless of how long the stimulus is applied. #### Why Other Options are Incorrect: * **B. Rate of discharge:** This refers to the frequency of action potentials, which is determined by the intensity of a suprathreshold stimulus and the refractory period, not the rheobase. * **C. Velocity of nerve conduction:** This is a measure of speed (distance/time) determined by myelination and axon diameter, unrelated to the initial threshold intensity. * **D. Specificity of impulse transmission:** This refers to the "all-or-none" law or synaptic pathways, rather than the electrical parameters of stimulation. --- ### High-Yield Facts for NEET-PG: * **Chronaxie:** The minimum **time** required for a current of **double the rheobase** strength to excite the tissue. * **Excitability Inverse Relationship:** Chronaxie is inversely proportional to excitability. A shorter chronaxie means the tissue is more excitable. * **Chronaxie Values:** Nerve (0.1 ms) < Skeletal Muscle (0.25 ms) < Cardiac Muscle (2.0 ms) < Smooth Muscle (high). * **Utilization Time:** The minimum time required to excite a tissue using a current exactly at the rheobase strength. (Note: This is less clinically useful than chronaxie because it is difficult to measure accurately).

Question 247: Which of the following has the least conduction velocity?

A. Somatic sensory neuron (Correct Answer)
B. Somatic motor neuron
C. Autonomic sensory neuron
D. Autonomic motor neuron

Explanation: To understand conduction velocity, we must refer to the **Erlanger-Gasser classification** of nerve fibers. Conduction velocity is directly proportional to the **fiber diameter** and the presence of **myelination**. ### **Explanation of the Correct Answer** The question asks for the **least** conduction velocity among the given options. * **Somatic sensory neurons** include **Type C fibers**, which carry sensations of slow pain, temperature, and crude touch. Type C fibers are **unmyelinated** and have the smallest diameter (0.4–1.2 μm), resulting in the slowest conduction velocity (0.5–2.0 m/s). * *Note:* While some somatic sensory fibers (like Type Aα for proprioception) are fast, the category "Somatic sensory" encompasses the slowest fibers in the human body (Type C). ### **Analysis of Incorrect Options** * **B. Somatic motor neurons:** These are **Type Aα fibers**. They are the thickest and most heavily myelinated fibers, boasting the highest conduction velocity (70–120 m/s). * **C. Autonomic sensory neurons:** These are generally Type B or Type C fibers. However, in the hierarchy of the Erlanger-Gasser scale, the unmyelinated somatic C-fibers (dorsal root C) are the standard for the slowest velocity. * **D. Autonomic motor neurons:** These consist of **Pre-ganglionic (Type B)** and **Post-ganglionic (Type C)** fibers. Type B fibers are myelinated and faster than Type C. While post-ganglionic fibers are slow, "Somatic sensory" is the preferred answer in competitive exams when referring to the slowest pain-conducting C-fibers. ### **High-Yield Clinical Pearls for NEET-PG** * **Order of Velocity:** Aα > Aβ > Aγ > Aδ > B > C (Slowest). * **Susceptibility to Blockade:** * **Local Anesthetics:** Type C fibers are blocked first (Small diameter). * **Pressure:** Type A fibers are blocked first (Large diameter). * **Hypoxia:** Type B fibers are blocked first. * **Type Aδ** is responsible for "fast pain" (sharp/localized), while **Type C** is responsible for "slow pain" (dull/aching).

Question 248: The 'all or none' law is obeyed by which of the following potentials?

A. Postsynaptic potential
B. Non-propagated potential
C. Action potential (Correct Answer)
D. Spike potential

Explanation: ### Explanation The **'All-or-None' Law** states that if a stimulus is strong enough to reach the threshold potential, a full response (action potential) is triggered; if the stimulus is sub-threshold, no response occurs. The magnitude of the response is independent of the strength of the stimulus once the threshold is reached. **1. Why Action Potential is Correct:** The **Action Potential (AP)** is a propagated, regenerative electrical signal. Once the threshold voltage (usually -55mV) is reached, voltage-gated sodium channels open in a positive feedback loop (Hodgkin cycle), resulting in a response of maximal amplitude. Increasing the stimulus intensity further will not increase the size of the AP, though it may increase the frequency. **2. Why the Other Options are Incorrect:** * **Postsynaptic Potential (PSP):** These are **graded potentials** (Excitatory or Inhibitory). Their magnitude depends on the amount of neurotransmitter released and the number of receptors activated. They do not obey the all-or-none law. * **Non-propagated Potential:** These include local potentials or electrotonic potentials. They stay localized, decay with distance, and their amplitude is proportional to the stimulus strength (graded). * **Spike Potential:** While a "spike" is often used synonymously with an action potential in nerve fibers, in the context of this specific question, "Action Potential" is the more definitive physiological term for the phenomenon that strictly obeys the all-or-none law across all excitable tissues (nerve and muscle). **High-Yield NEET-PG Pearls:** * **Exceptions:** The all-or-none law applies to a **single** nerve fiber or muscle fiber. It does **not** apply to a whole nerve trunk or a whole skeletal muscle (due to recruitment of multiple motor units). * **Graded Potentials:** Examples include Receptor potentials, End-plate potentials (EPP), and Pacemaker potentials. * **Refractory Period:** The all-or-none law is the reason why action potentials cannot be summed, unlike graded potentials.

Question 249: All of the following statements regarding activity at the neuromuscular junction are true EXCEPT:

A. Action potentials on the alpha motor neuron release acetylcholine (Ach) from the axon terminal.
B. Ach stimulates muscarinic cholinergic receptors on the muscle endplate. (Correct Answer)
C. Cholinergic receptors on the muscle endplate are cation channels.
D. Alpha motor neurons excite, and Renshaw cells inhibit, the muscle endplate.

Explanation: ### Explanation The neuromuscular junction (NMJ) is a specialized synapse where a motor neuron communicates with a skeletal muscle fiber. Understanding the specific receptors and neurotransmitters involved is crucial for NEET-PG. **1. Why Option B is the Correct Answer (The False Statement):** In skeletal muscle, Acetylcholine (ACh) acts on **Nicotinic (N$_m$) receptors**, not muscarinic receptors. Nicotinic receptors are **ionotropic** (ligand-gated ion channels), allowing for rapid depolarization. Muscarinic receptors are G-protein coupled receptors found primarily in the autonomic nervous system (e.g., heart, smooth muscle, glands). **2. Analysis of Other Options:** * **Option A:** True. Depolarization of the alpha motor neuron opens voltage-gated calcium channels, triggering the exocytosis of ACh vesicles into the synaptic cleft. * **Option C:** True. The N$_m$ receptor is a non-specific **cation channel**. When ACh binds, the channel opens, allowing an influx of Na$^+$ (and some efflux of K$^+$), leading to the End Plate Potential (EPP). * **Option D:** True. Alpha motor neurons are the "final common pathway" for muscle excitation. **Renshaw cells** are inhibitory interneurons in the spinal cord that provide recurrent inhibition to alpha motor neurons (via glycine), preventing over-excitation. **Clinical Pearls & High-Yield Facts:** * **Myasthenia Gravis:** Antibodies against the post-synaptic **Nicotinic (N$_m$) receptors**. * **Lambert-Eaton Syndrome:** Antibodies against pre-synaptic **Voltage-Gated Calcium Channels (VGCC)**. * **Botulinum Toxin:** Inhibits ACh release by cleaving SNARE proteins. * **Curare:** A competitive antagonist that blocks Nicotinic receptors at the NMJ, causing paralysis.

Question 250: Repolarization of a nerve is due to:

A. Hydrogen ions
B. Potassium ions (Correct Answer)
C. Sodium ions
D. Calcium ions

Explanation: **Explanation:** The action potential of a nerve fiber consists of two primary phases: depolarization and repolarization. **Repolarization** is the process by which the membrane potential returns to its negative resting state after depolarization. This is primarily achieved by the **efflux (outward movement) of Potassium ions ($K^+$)**. During this phase, voltage-gated $Na^+$ channels close (inactivate), and voltage-gated $K^+$ channels open. Since the concentration of $K^+$ is higher inside the cell, these ions rush out, removing positive charges from the interior and restoring the electronegativity of the cell. **Analysis of Incorrect Options:** * **Sodium ions ($Na^+$):** These are responsible for **Depolarization**. The rapid influx of $Na^+$ into the cell makes the interior positive. * **Calcium ions ($Ca^{2+}$):** While $Ca^{2+}$ is crucial for neurotransmitter release at the synapse and the "plateau phase" in cardiac muscle action potentials, it is not the primary ion responsible for repolarization in nerve fibers. * **Hydrogen ions ($H^+$):** These ions influence the pH of the environment but do not play a direct role in the ionic flux of a standard nerve action potential. **High-Yield NEET-PG Pearls:** * **Resting Membrane Potential (RMP):** Primarily maintained by $K^+$ "leak" channels (RMP of a typical neuron is -70 mV). * **Hyperpolarization:** Occurs when $K^+$ channels remain open slightly longer than necessary, moving the potential closer to the $K^+$ equilibrium potential (-94 mV). * **Na+-K+ ATPase Pump:** Does not cause repolarization but restores the ionic gradients (3 $Na^+$ out, 2 $K^+$ in) *after* the action potential cycles are complete.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

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Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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